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The human skeletal system is comprised of individual bones and cartilage that receive a supply of blood and are held together by fibrous connective tissue, ligaments, and tendons. The three main functions of the skeletal system are protection, motion, and support. The system protects the body by enclosing the vital organs, it permits locomotion by responding at certain joints to the contractile activities of skeletal muscles, and it supports the body by serving as a framework to which tendons and fascia are attached, enabling skeletal muscles, viscera, and skin to obtain a holdfast. It also serves as a depot for calcium, which is vital to proper functioning of cell membranes, and for phosphorus, which is needed in intermediary metabolism. In addition, the skeletal system is important because bone produces blood cells.
At birth the human body has about 275 bones, but as the body develops many of these bones fuse together. In the adult human the skeleton consists of 206 name-bearing bones and a variable number of largely unnamed sesamoid bones. Sesamoid bones develop in the capsules of certain joints or in tendons, which hold muscle to bone, where they provide special support or reduce friction. The best-known sesamoids are the patella (kneecap) and pisiform (wrist bone).
Classification Of Bones
Bones may be classified as long, short, flat, irregular, or sesamoid. The long bones are those of the limbs, except for the wrist, ankle, and kneecap. They consist of a central shaft (diaphysis) between two ends (epiphyses) that form joints with one or more other bones. The short bones are in the wrists and ankles. They consist of a spongy core within a shell of compact bone. The flat bones include the ribs and many skull bones. They consist of two plates of compact bone with a spongy layer, called a diploe, between. All the remaining named bones are irregular bones, except the patella, pisiform, and some bones in the feet.
Divisions Of The Skeletal System
The skeleton is comprised of an axial and an appendicular division. The axial division consists of the skull, spinal column, and thoracic cage, whereas the appendicular division includes an upper extremity (shoulder girdle and its paired appendages) and a lower extremity (pelvic girdle and its paired appendages).
The Skull
The skull consists of the cranium, which houses the brain, and the face. The cranium is made up of eight cranial bones, including one frontal, two parietal, two temporal, one ethmoid, one occipital, and one sphenoid.
The frontal bone is the forehead. Its supraorbital margins, which can be felt through the skin of the eyebrows, provide a certain degree of protection to the eyes. Present within this bone, directly over the eyes, is the frontal sinus, which opens directly into the nasal cavity.
The paired parietal bones comprise the walls of the cranium, whereas the paired temporals form the sides and part of the base. Each temporal bone encloses an ear and forms a movable joint with the lower jaw. In addition to enclosing the cochlea for hearing, the temporal bone encloses the labyrinth, which contains special sensory receptor cells (proprioceptors) for body balance.
The ethmoid bone lies mainly within the skull cavity, between the cranial and nasal cavities. It contains a sievelike plate (a cribriform plate) through which nerve fibers from olfactory receptors pass en route to the brain. A front view of the ethmoid bone reveals a perpendicular septum (partition), which separates the nostrils from one another. Present on each side of the ethmoid bone are ethmoid sinuses that drain into the nasal cavity.
The occipital bone forms the floor and lower rear wall of the cranium. Its outstanding feature is the foramen magnum, a large hole through which the spinal cord passes into the brain. Adjacent to the foramen magnum are two protuberances, called occipital condyles, which articulate with the vertebral column and allow the head to move up and down, as in nodding.
The sphenoid (sphinxlike) bone lies immediately in front of (anterior to) the temporal bones. A septum down the midline divides the sphenoid sinuses that drain into the nasal cavity. A large depression, called a sella turcica, is within what might be viewed as the ceiling of the sphenoid bone. This provides protection to the pituitary gland, which is suspended immediately above it.
Three paired bones, called the auditory ossicles, are considered part of the cranium of the skull. Present within each middle ear within the temporal bone are a malleus, incus, and stapes. These ear bones transmit sound waves into the inner ear.
The Hyoid Bone
Another bone that may be grouped with the skull bones is the hyoid (U-shaped) bone. Although this bone is located in the neck, it is suspended by ligaments from the styloid (spear-shaped) processes that extend downward from the temporal bones. It functions as a point of attachment for most of the anterior muscles that lie in the neck.
The Face
The face contains 14 facial bones. Seven are superficial and can be felt with the fingers, whereas the other seven are located deeply. The deep facial bones include two lacrimals, two palatines, one vomer, and two inferior conchae. The lacrimal bones form part of the median wall of each orbit (eyeball cavity). The palatine bones form part of the lateral walls and floor of the nasal cavity, part of the roof of the mouth, the floor of the orbits, and the rear (posterior) portion of the hard palate. The vomer, which is flat, forms the rear (posterior) and basal portion of the nasal septum.
The superficial facial bones include two nasal, two zygomatic, two maxillary (upper jaw), and one mandibular (lower jaw). The nasal bones lie side by side, uppermost along the median line of the nose. They support the flexible cartilaginous inferior region of the nose. The zygomatic bones form the prominences of the cheek and part of the lateral and inferior walls of the orbits. The maxillary bones fuse with one another medially to form the upper jaw, which contains alveoli (sockets) for embedding the teeth. The maxillary bones constitute the bulk of the hard palate in the roof of the mouth. In addition, they form part of the walls of the orbital and nasal cavities. In the course of development, should they fail to fuse medially within the mouth, a deformity known as a cleft (split) palate appears. Like the frontal, ethmoid, and sphenoid bones, the maxillary bones contain sinuses that drain into the nasal cavity.
The Spinal Column
The spinal column of most adults consists of 26 bony segments (vertebrae): 7 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 1 sacral, and 1 coccygeal (tail) bone with multiple segments. Each vertebra, except the first and last, has a so-called body. These bodies are aligned with one another and are separated from each other by an intervertebral disk.
The vertebral bodies function to bear weight, as in standing or sitting. The disks, made of a fibrous elastic cartilage, cushion the vertebrae, lubricate the joints between vertebrae, and act as shock absorbers. A pair of stalks, called pedicels, arises laterally from each vertebral body. The pedicels of one vertebra lie adjacent to those of another vertebra. They are arranged to form an opening through which the spinal nerves emerge.
In addition to a vertebral body and pedicels, all vertebrae except the coccyx possess a vertebral foramen (opening) through which the spinal cord passes, and various processes that, together with their attached ligament and muscle, limit the kinds of movements the spine can make.
The Thoracic Cage
The thoracic cage is composed of 12 pairs of ribs, 12 thoracic vertebrae, and the sternum (breastbone). The first 7 pairs of ribs are called true ribs because they form a direct union with the sternum. Ribs 8 through 12 are called false ribs because they are not attached to the sternum. Instead they are united with each other and with rib 7. Ribs 11 and 12 are called floating ribs because they have no anterior attachment.
The Shoulder Girdle
The shoulder girdle consists of two scapulae (shoulder blades) and two clavicles (collarbones). The entire apparatus, together with its musculature, permits raising the hands skyward and other functions that most animals except primates are incapable of performing.
The Upper Paired Appendages
Each of the upper paired appendages consists of an arm, forearm, and hand. The arm contains a single bone, called a humerus. The head of the humerus forms a ball-and-socket joint with the scapula by way of a large depression in the scapula known as the glenoid cavity, or fossa.
The forearm contains two bones, a radius and an ulna. The hand contains 8 carpals, 5 metacarpals, and 14 phalanges. The carpals constitute the wrist. The metacarpals, whose distal ends are known as knuckles, form the palm. Distal to the metacarpals are the phalanges, two for the thumb and three for each finger.
The Pelvic Girdle
The pelvic girdle supports the trunk on the thighs while standing, permits sitting, and provides protection to the urinary bladder, ovaries, oviducts and uterus, and rectum. It is formed from two hipbones, a sacrum, which lies between the hipbones, and a coccyx, or tailbone. Each hipbone, or innominate bone, consists of an ilium, ischium, and pubis, which are ossified in adults. The pubis forms the front (anterior) region of the hipbone, where the two hipbones unite by way of a cartilaginous bridge known as a pubic symphysis. Where the three components of each hipbone unite, a socket is formed. Known as an acetabulum, this socket receives the head of the femur (thigh).
The Lower Paired Appendages
Each lower paired appendage is composed of a thigh, leg, and foot. The thigh is built upon the femur, a large heavy bone that connects the hipbone to the tibia of the leg. The joint between the femur and tibia is provided with some protection by the patella (kneecap), which is embedded in the tendon of the large thigh muscles (quadriceps).
In addition to a tibia, the leg contains a fibula. Whereas the tibia (shinbone) serves to bear weight, the fibula facilitates certain movements at the ankle, which is the joint between the tibia, the fibula, and one of the tarsal bones, the talus.
The foot contains 7 tarsal bones, 5 metatarsals, and 14 phalanges. Together with surrounding tissues, these bones form a longitudinal arch from heel to toes and a metatarsal arch from side to side.
Leukocytes, erythrocytes, and blood platelets are all ultimately derived from unspecialized cells in the bone marrow. Lymphocytes produced in the bone marrow seed the thymus, spleen, and lympg nodes, producing self-replacing lymphocite collonies in these organs. It could be specifically said that the bone marrow manufactures our bodies immune system or at least the cells that are required for it (immune system).
The epiphyseal plate, or growth plate, is a band of cartilage located near the distal end of a long bone. Cartilage cells within the epiphyseal plate divide and form new cartilage cells. These cells are replaced by bone at the bottom of the epiphyseal plate, allowing the plate to continue to grow at the distal end. Growth continues until the bone reaches its full lenght, at which time bone has replaced all the cartilage, including the cartilage in the epiphyseal plate. After this, no further longitudial growth is possible in the bone.
Ossification is the process by which bones develop. There are two types of ossification - the conversion of cartilage into bone and the formation of membrane layers.
Cartilage is a tough, flexible connective tissue. During the first month of human fetal devalopment the entire skeleton is made up of cartilage. Cartilage is composed of elastine, collagen and organic material. During the second month of fetal devalopment osteocytes begin to devalop in this cartilage. The osteocytes release minerals that lodge in empty spaces between the cartilage cells. Eventually most of the cartilage is replaced by bone. However, some cartilage remains between bones, at the end of the nose, in the external ears, and on the inside of the tracheae. It is the cartilage that makes these areas flexible.
A few bones, such as those in the clavicle and in some parts of the skull, devalop directly into hard, nonmembranous bone from membranous connective tissue without a distinctive cartilaginous phase. The membranes of the tissue ossify and become flat plates of bone. For example, the membranes in the head are replaced by the long plates that form the skull. The plates have suture lines between them. Bones are composed of hydroxiapatite, collagen and inorganic material. Bones continue to devalop after birth.
The removal of certain substances ( stated in the paragraph) from a bone may cause it to become flexible as seen in the movie during class. If these materials could be theoretically added to the bone again they would become harder again.
The bones are all smoothly jointed and firmly held together by flexible ligaments that keep the bones aligned during movement. The ends of the bones in each typical joint are padded with cartilage, covered with a thin sheath called the synovial membrane, and oiled with a lubricating, or synovial, fluid so that they can be used constantly and yet be protected against wear and tear. The degree of movement possible in a joint varies. Joints, therefore, are classed as immovable, yielding, or having free motion. For example, the joints of the cranium are immovable; the vertebrae are yielding; and the shoulder joint has free motion. The muscles in general are attached to the bones across the joints so that movements are brought about by the shortening, or contraction, of opposing pairs of muscles.
Although arthritis is defined as "joint inflammation," the term is used loosely to include the approximately 100 inflammatory and noninflammatory diseases that affect the joints, connective tissue, and other supporting tissue. These diseases are also called rheumatic diseases or connective tissue diseases. The medical specialty concerned with these diseases is rheumatology. Causes of these disorders include infections, apparent abnormalities of the immune system, effects of injuries and aging, and in some cases hereditary predisposition. About one of seven Americans--40 million people--have some form of arthritis. In 40 percent of the cases, the disease is severe enough to require medical care. The personal and economic burden is enormous, with annual cost in lost wages and medical expenses of more than $15 billion. Of those requiring medical care, half of arthritis patients have osteoarthritis, one-quarter have rheumatoid arthritis, and the remainder have other related diseases.
Collagen disease is a term applied to a group of diseases that involve abnormalities of the immune system and inflammation of connective tissue and blood vessels. Collagen fibers are a major component of connective tissue. The name rheumatic disease is often used because the most common of all such diseases, rheumatoid arthritis, shows all of the characteristics of this group of diseases. The blood plasma of many patients with collagen disease shows significant levels of autoantibodies (antibodies against the body's own proteins or cells); the resulting antigen-antibody reaction leads to inflammation in many body tissues. Collagen diseases include rheumatoid ARTHRITIS, RHEUMATIC FEVER, LUPUS ERYTHEMATOSUS, dermatomyositis, polyarteritis nodosa, and scleroderma.
Both rheumatoid arthritis and rheumatic fever are characterized by widespread joint pain; rheumatic fever may also result in permanent heart damage. Lupus erythematosus is an autoimmune disease that affects the brain, joints, kidneys, skin, and membranes lining body cavities. Dermatomyositis is most commonly seen as a rash accompanied by muscular pain. In polyarteritis nodosa the walls of arteries are damaged, and in scleroderma thick layers of collagen fibers are deposited; both diseases result in impaired organ function.
A dislocation is any displacement of a body structure from its normal position, although it is usually a misalignment of bones at a joint. Bones are normally held together in proper alignment by tough fibrous bands called LIGAMENTS, which are attached to each bone, and by a fibrous sac called the articular capsule, also connected to the bone. These connecting structures are relatively inelastic, but they do allow joint movement within limits. A dislocation is usually caused by a violent movement at the joint that exceeds normal limits, tearing the ligaments and the articular capsule and throwing the bone or bones out of place. Dislocations most commonly occur at the shoulders, fingers, jaw, elbows, knees, and hips as a result of trauma, although they sometimes occur as a result of diseases that affect the joints.
In a bone dislocation the joint is immobile, and the affected limb may be locked in an abnormal position; FRACTURES may also be present. FIRST AID treatment consists of immobilizing the joint with a splint, sling, or bandage.
Rheumatism is a nonspecific term for several diseases that cause inflammation or degeneration of joints, muscles, ligaments, tendons, and bursae. The term includes rheumatoid ARTHRITIS and other degenerative diseases of the joints; BURSITIS; fibrositis; gout; lumbago; myositis; rheumatic fever; sciatica; and spondylitis.
Palindromic rheumatism is a disease that causes frequent and irregular attacks of joint pain, especially in the fingers, but leaves no permanent damage to the joints. Psychogenic rheumatism is common in women between the ages of 40 and 70, although men also contract this disease. Symptoms include complaints of pain in various parts of the musculoskeletal system that cannot be substantiated medically. This condition can be alleviated by psychotherapy.
One of the common forms of rheumatism is rheumatoid arthritis, a disease of unknown cause that affects 1 to 3 percent of the population. This disease causes joint deformities and impaired mobility as a consequence of chronic inflammation and thickening of the synovial membranes, which surround joints. As the disease progresses it produces ulceration of cartilage in the joints. Rheumatoid arthritis usually occurs between ages 35 and 40 but can occur at any age. It characteristically follows a course of spontaneous remissions and exacerbations, and in about 10 to 20 percent of patients the remission is permanent.
Osteoporosis is a condition of bone characterized by excessive porosity, or bone tissue reduction. Absorption of old bone exceeds deposition of new bone; the result is an enlargement of spaces normally present and a thinning of the bone from the inside. No change occurs in the outside dimensions, except in compression of weight-bearing bones. Senile and postmenopausal, or primary, osteoporosis, the most common type, is found only in elderly persons and in women who have passed through menopause. It is characterized by compression of the vertebrae with resultant back pain and loss of height, and by susceptibility to fractures. Disuse, or secondary, osteoporosis involves bones that have been immobilized by paralytic disease or traumatic fractures or have been subjected to prolonged weightlessness during spaceflight. Other osteoporoses are associated with endocrine diseases and with nutritional disorders such as anorexia nervosa. Exercise programs and calcium supplements are used in prevention and treatment, and slow-release fluorine tablets have been developed for spinal osteoporosis. Older women may receive estrogen therapy but with increased risk of uterine cancer.
